Field of the Invention
The invention relates to a printing unit that includes a rotatably mounted transport device for transporting a print carrier sheet. The transport device is equipped with a holding device for holding the print carrier sheet. The printing unit includes a NIP (Non-Impact-Printing) printing head that is aligned with the transport device and is intended for printing the print carrier sheet.
Published German Patent Application DE 197 04 003 A1 describes a printing unit with a transport device designed as an impression cylinder mounted rotatably about its cylinder axis. The holding device of the transport device is designed as a gripper row. The NIP printing head of this printing unit may be a laser printing head or an ink-jet printing head. One disadvantage of the printing unit is that the gripper row permanently projects well beyond the circumferential contour of the impression cylinder. Consequently, the size of the printing nip, that is to say the distance of the NIP printing head from the circumferential surface of the impression cylinder, can be reduced only to the limit which ensures that the gripper row can run through the printing nip without colliding with the NIP printing head.
It is desirable, however, for the NIP printing head to be capable of being advanced beyond this limit nearer to the print carrier sheet, in order to achieve a better pixel geometry of the pixels that are produced on the print carrier sheet with the NIP printing head.
Published German Patent Application DE 199 01 698 A1 likewise describes a printing unit that corresponds to the generic type initially mentioned, in which the transport device is a transport belt that rotates about two deflecting rollers and the holding device is a prestressed gripper tongue. By means of the very flat form of the gripper tongue which can be manufactured, for example, from spring plate, the printing nip can be reduced to a dimension of less than 1 mm. Although a satisfactory pixel quality can be expected, designing the holding device as a gripper tongue is undesirable because of the comparatively low clamping force that is exerted on the print carrier sheet by the gripper tongue. To be precise, the clamping force is determined by the prestress of the gripper tongue, which, in turn, depends on the thickness, for example the spring plate thickness, of the gripper tongue. In order to increase the prestress and the clamping force, therefore, the thickness of the gripper tongue would have to be increased. Consequently, it will be necessary to increase the size of the printing nip, in order to prevent the thickened gripper tongue from colliding with the NIP printing head. Increasing the size of the printing nip would, in turn, be detrimental to the pixel quality, so that the desired aim would not be fulfilled.
It is accordingly an object of the invention to provide a printing unit which overcomes the above-mentioned disadvantages of the prior art apparatus of this general type.
In particular, it is an object of the invention to provide a printing unit that allows the NIP printing head to be in close proximity with the transport device.
With the foregoing and other objects in view there is provided, in accordance with the invention, a printing unit that includes: a rotatably mounted transport device for transporting a print carrier sheet; and a non-impact-printing printing head for printing the print carrier sheet. The printing head is aligned with the transport device. The transport device is formed with an interior. The transport device includes a holding device for holding the print carrier sheet, and the holding device is adjustably mounted to enable movement of the holding device into the interior of the transport device.
In accordance with an added feature of the invention, the holding device is a gripper system.
In accordance with an additional feature of the invention, the gripper system is opened when the gripper system is in the interior of the transport device.
In accordance with another feature of the invention, a cam mechanism is assigned to the holding device for moving the holding device into the interior of the transport device.
In accordance with a further feature of the invention, the transport device includes a suction device for sucking up the print carrier sheet.
In accordance with a further added feature of the invention, the transport device has a sheet bearing surface; and the suction device has suction orifices that are arranged in rows and that are formed in the sheet bearing surface of the transport device.
In accordance with a further additional feature of the invention, a suction-air source is connected to the suction device.
In accordance with yet an added feature of the invention, the transport device is a transport cylinder.
In accordance with yet an additional feature of the invention, the printing head is an ink-jet printing head.
With the foregoing and other objects in view there is provided, in accordance with the invention, a sheet-fed rotary printing machine, that includes: at least one first printing unit having an impression cylinder and cylinder selected from the group consisting of a rubber-blanket cylinder and a printing-form cylinder; and a second printing unit. The second printing unit includes: a rotatably mounted transport device for transporting a print carrier sheet; and a non-impact-printing printing head for printing the print carrier sheet. The printing head is aligned with the transport device. The transport device is formed with an interior. The transport device includes a holding device for holding the print carrier sheet, and the holding device is adjustably mounted to enable movement of the holding device into the interior of the transport device.
The holding device can thus automatically be adjusted completely into the interior of the transport device shortly before the passage of the NIP printing head and out of the interior again shortly after the passage. This automatic retraction and reemergence of the holding device may take place periodically, that is to say once during each revolution of the transport device. By the holding device being drawn into the transport device, the holding device is adjusted away from the NIP printing head, so that, even when the distance of the NIP printing head from the transport device is very slight, a collision of the holding device with the NIP printing head is absolutely ruled out.
The holding device may be a holding device, for example a suction strip, retaining the print carrier sheet at its leading edge by means of electrostatic or pneumatic attraction.
The NIP printing head may operate according to an electrophotographic NIP (Non-Impact-Printing) method.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a printing unit it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.